ByKate Brumback, Associated PressSeptember 9, 2014

A person wearing a haz-mat suit steps out of an ambulance as an Ebola patient arrives for treatment, Tuesday, Sept. 9, 2014, in Atlanta. The fourth American aid worker sickened with the Ebola virus arrived Tuesday morning for treatment at Emory University Hospital, where two others have been successfully treated.

Atlanta — The fourth US aid worker sickened with Ebola arrived in Atlanta on Tuesday, mirroring the carefully choreographed routine of the two Americans already successfully treated and released from Emory University Hospital's special isolation unit.

The latest patient — unidentified by hospital officials, who cited privacy restrictions — arrived more than a month after missionaries Dr. Kent Brantly, 33, and Nancy Writebol, 59. All were flown in the same specially equipped medical plane from Africa to a military air base just outside Atlanta, then taken in ambulances with police escorts to the hospital. Helicopters buzzed above, and reporters and television cameras gathered in clusters along the street. The patients wore bulky, protective suits. On Tuesday, Emory's third patient walked from the ambulance, as Brantly did. Writebol was taken in a stretcher.

The delicate process seemed to run like a well-oiled machine, attracting fewer curious onlookers and drawing less concern from the public the third time around.

"Those of us who are at Emory, we're not concerned because we know the quality of Emory medical care, and we know the reason they were brought here is because Emory is capable of containing it and treating them," law student Grace Van Dyke said.

Dr. Aneesh Mehta said the fact that the patient walked is a good sign but emphasized that doctors will look at overhaul health and all options for a treatment plan. Mehta said the medical team's confidence was bolstered by the first two patients' recovery: They were released last month, and doctors stressed that they showed no sign of the virus, posing no public health risk.

"We have even more confidence that our advanced supportive care measures did have an impact, so those are going to be the mainstay of our care," Mehta said.

He did not release other details of the patient's identity or status. But the World Health Organization said a doctor who had been working in an Ebola treatment center in Sierra Leone tested positive for the disease and was to be evacuated Monday in stable condition. The Ebola outbreak sweeping West Africa has killed more than 2,200 people and has taken a particularly high toll on health care workers.

Dr. Rick Sacra, 51, of Worcester, Massachusetts, is being treated at the Nebraska Medical Center in Omaha. His condition is improving, one of his doctors said Tuesday.

Federal officials said they asked the Nebraska hospital to treat him instead of Emory to prepare other isolation units for more Ebola patients if needed. The Nebraska's unit has 10 beds. Emory's isolation unit is ready to hold three people, and that capacity could be increased, Mehta said.

Mehta did not say which, if any, drugs would be used on the latest patient. Brantly and Writebol were the first to get the experimental drug ZMapp, but the supply is gone. Both credited the drug with helping their recovery, though there is no way to know its effects. Sacra is being treated with a different experimental drug. His doctors have refused to name it but say they've been consulting with experts on Ebola.

Once a new batch of ZMapp is ready, it needs basic tests before it can be tried again, officials have said.

In addition, a US-created vaccine began a safety study last week at the National Institutes of Health. A study of a second experimental vaccine — this one created in Canada — will begin "any day now" the Walter Reed Army Institute of Research, Dr. David Hone of the Defense Threat Reduction Agency said Tuesday.